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Beneficiation of Phosphates VII Proceedings

Spring 4-1-2015

Understanding the comminution mechanism ofhigh-pressure grinding rolls: Lower cost, higherefficiency and selectivityFrancisco SotilloPerUsa EnviroMet Inc

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Recommended CitationFrancisco Sotillo, "Understanding the comminution mechanism of high-pressure grinding rolls: Lower cost, higher efficiency andselectivity" in "Beneficiation of Phosphates VII", P. Zhang, FIPR; J. Miller, Univ. of Utah; L. Leal, Univ. of Sao Paolo; A. Kossir, OCPGroup; E. Wingate, Worley-Parsons Services Pty Ltd. Eds, ECI Symposium Series, (2015). http://dc.engconfintl.org/phosphates_vii/18

UNDERSTANDING THE COMMINUTION MECHANISM

OF HIGH-PRESSURE GRINDING ROLLS: Lower Cost,

Higher Efficiency and Selectivity

BY

Dr. Francisco J. Sotillo

PerUsa EnviroMet Inc.

228 Pinellas Street

Lakeland, FL 33803-4832

USA

CONTENT

•Introduction

•Concepts

•Characterization Studies

•PerUsa’s Model – INSTRON Tests

•Examples of Applications

•Summary and Conclusions

1

INTRODUCTION

• Schematic diagram of a High Pressure Grinding Roll (HPGR).

2

INTRODUCTION (CONT.)

• The technology used in the High Pressure Grinding Roll

(HPGR) has demonstrated that:

‾ Constitutes the only real development, and the most

significant innovation of the 20th Century in comminution.

‾ Applies a Bed of Particles Comminution Mechanism,

allowing selective grinding.

‾ Reduces the total energy consumption by 10% to 50% of

that consumed by tumbling mills (rod and ball mills, etc.).

‾ Increases grinding throughput by 15% to 30% with respect

to that of conventional comminution systems.

‾ Occupies less floor space (smaller plant print).

3

INTRODUCTION (CONT.)

• The technology used in the High Pressure Grinding Roll (HPGR) has demonstrated that:

‾ Has similar or lower CAPEX than other comminution technologies.

‾ Reduces iron consumption (lower wear rate) by using different rolls profiles according to the type of ore process, and the operating conditions.

‾ Operates under a wide range of ore moisture content, up to 30% moisture.

‾ Produces a wide particle size distribution (PSD) with relative high content of fines.

‾ Is easy to scale-up to grind hundred of tons of solids per hour.

4

CONCEPTS

• Traditionally, crushing and grinding are unit operations that are non-

selective since minerals are broken by applying pressure using

comminution surfaces (metallic plates, steel rods and balls, etc.).

• Under these conditions, the particles are loaded predominantly in

compression and fails in tension.

• As the particle is subjected to an increasing stress, existing flaws

intensify the applied stress, and particles fails at the largest flaw.

• This process is subjected to probabilities; a1, for being submitted to

compression by grinding media; a2, of having crystallographic

defects or flaws; a3, of being in the right position; a4, of being

broken by the applied compression.

• Thus, it is the least selective and the lowest efficient

comminution mechanism.

5

CONCEPTS (CONT.)

• The most efficient comminution mechanism is that of individual particle grinding, but it has not been possible to be applied to industrial grinding yet.

• Grinding in a Bed of Particles is the second most energy efficient and selective mechanism, and it is applicable to industrial comminution.

• High Pressure Grinding Rolls (HPGR) operate under this mechanism of Bed of Particles Comminution that allows selective grinding of different mineralogical species in the ore.

• In a HPGR, grinding in a Bed of Particles Mechanism depends on the:

− Strength of the material.

− Internal friction between particles.

− Friction with the machine surfaces.

− Type and size distribution of the feed material.

− Surface moisture content.

− Grain borders between different mineralogical species in the feed material.

6

CONCEPTS (CONT.)

• Therefore, applying the mechanism of Bed Particles Comminution

in a HPGR resulted in:

− The gap of the HPGR only defining the maximum particle size

to be produced.

− Selective grinding of the soft mineral species by the hard mineral

species in an autogenous grinding system.

− High fraction of the energy applied to the HPGR is consumed in

grinding since the particles are hold in a pseudo-confine space.

− Residual stresses and micro-cracks that reduce energy

consumption in downstream attrition scrubbing, comminution,

and enhance leaching, etc.

7

CONCEPTS (CONT.)

8

• Schematic of particle bed regions between rolls in a HPGR.

CONCEPTS (CONT.)

• Thus, HPGR is chocked-fed at not less than 50 MPa to maintain a

Bed of Particles Comminution Mechanism.

• Recent Studies on Crushing, and rod and ball mill grinding have

pointed out the importance of developing practical methods of

enhancing liberation of minerals from each other, mainly by using

inter-particles breakage occurring in particle bed comminution

process.

• New jaw and cone crushers are operated in such a way that the

crushing chamber could develop a bed of particles comminution

area before discharging the crushed material.

9

CHARACTERIZATION STUDIES

• Florida pebble samples were representative of different types of high-dolomite phosphate pebbles with MgO content of 0.69% to 3.15%, and the Australian Phosphate samples were representative of sedimentary phosphate ores with high-clay content, 4.10% and 11.87%

• The pebble samples exhibit a mean particle size in the range of 1.63 mm for Mine I, 1.94 mm for Mine II, 1.72 mm for Mine III, and 1.37 mm for Mine IV; whereas, the Australian Phosphate ores showed 0.150 mm and 1.69 mm for the ultra-high-clay and high clay ores, respectively.

• The chemical analyses of the high-dolomite phosphate ores accounted for 96% to 99% of the mineralogical species, and those of the Australian phosphate ores accounted for 99%.

10

CHARACTERIZATION STUDIES (CONT.)

11

Florida Pebbles Australian Ores

Data Mine I Mine II Mine III Mine IV Ultra-High-Clay High-Clay

Chemical Analysis

P2O5, % 23.24 26.60 22.76 24.39 6.04 20.80

MgO, % 0.69 1.53 3.15 1.64

Al2O3, % 11.87 4.10

Insol, % 26.98 13.06 17.29 19.68 63.01 46.00

Moisture, % 9.88 9.60 11.60 8.93 8.50 8.00

Physical Properties

Specific Gravity 2.56 2.66 2.57 2.76 2.69 2.70

Angle of Repose, degree 44.58 41.00 48.08 36.83 35.00 35.00

d50 (mean size), mm 1.63 1.94 1.72 1.37 0.15 1.69

Mineralogical Analysis

Francolite, % 66.12 76.24 65.22 69.91

Apatite, % 21.50 67.51

Dolomite, % 1.77 5.24 12.92 5.90

Kaolinite, % 35.03 12.10

Quartz, % 26.98 13.06 17.29 19.68 41.00 16.30

Total Minerals, % 99.23 96.01 98.13 97.53 98.90 99.10

PERUSA’S MODEL – INSTRON TESTS

• The precursor PerUsa’s Model-INSTRON Tests were aimed at

determining, on a small scale, if selective grinding occurs.

• These tests were carried out only on Florida High-dolomite Phosphate

pebbles, providing information regarding the energy absorbed,

mechanical behavior (relaxation tests), effect on the particle size

distribution, and P2O5 and MgO distributions.

• INSTRON Tests results followed by screening at 106 µm demonstrated

that it was possible to selective grind dolomite from each of the four

Florida mines.

• P2O5 grade was between 22.2% and 26.3% with MgO grade between

0.53% to 1.08%, resulting in P2O5 recovery between 83% to 88% and

MgO rejection between 41% to 82%.

12

PERUSA’S MODEL - INSTRON TESTS (CONT.)

13

Florida Pebbles

Data Mine I Mine II Mine III Mine IV

Feed

P2O5, % 23.24 26.60 22.76 24.39

MgO, % 0.69 1.53 3.15 1.64

Ground Product (+150 M)

d50 (mean size), mm 0.93 0.73 0.65 0.74

P2O5, % 22.82 26.19 25.52 24.72

MgO, % 0.53 0.80 0.75 1.08

P2O5 Recovery, % 87.75 84.64 87.48 83.15

MgO, Rejection, % 41.11 52.29 81.87 44.99

Energy Abs, Em, KWh/ton 0.41 0.72 0.69 0.82

Mechanical Behavior Elasto-Plastic Elasto-Plastic Elasto-Plastic Elasto-Plastic

PERUSA’S MODEL - INSTRON TESTS (CONT.)

14

0

5

10

15

20

25

30

35

40

0 20 40 60 80 100 120 140 160 180 200

CU

MU

LA

TIV

E F

INE

S (

t10),

per

cen

t

APPLIED PRESSURE, MPa

INSTRON TEST RESULTS

Mine I Mine II Mine III Mine IV

• The Mean Particle size (d50) decreases as the Applied Pressure increases;

thus, the Cumulative Fines (t10) increasing. Cautiously, due to grinding in a

confine space, it could be infer that Mine IV is the hardest phosphate

pebble (rejected of heavy media plant), followed by Mine II, Mine I, Mine

III being the softest.

PERUSA’S MODEL - INSTRON TESTS (CONT.)

15

• INSTRON Tests demonstrated that selective grinding of dolomite in a

bed of particles takes place liberating it. For Example for Mine III,

liberation of dolomite did not increased after 100 MPa of applied pressure

due to phosphate pebbles starting to grind, resulting in 87.5% P2O5

recovery and 81.9% MgO rejection .

0

10

20

30

40

50

60

70

80

90

100

0 20 40 60 80 100 120 140 160 180 200

P2O

5 R

EC

OV

ER

Y A

ND

MgO

RE

JE

CT

ION

,

per

cen

t

APPLIED PRESSURE, MPa

MINE III - VALUES DISTRIBUTION - MESH OF SEPARATION 150

P2O5 Recovery MgO Rejection

EXAMPLES OF APPLICATIONS • Even though the application of HPGR has just start to be looked at in

the Phosphate Industry, it is a mature technology in other Mineral Industries.

• Based on the understanding of the Bed of Particles Comminution Mechanism, it is clear that:

− HPGR is not only a size reduction piece of equipment with great advantages on water and energy consumption.

− HPGR must be tied to a suitable classification, desliming, and/or additional comminution unit operations to take full advantage of this technology (selective grinding).

− HPGR products with better liberation of mineral species enhanced separation processes performance, such as flotation.

− HPGR comminution residual stresses and micro-cracks production reduce the energy consumption in downstream comminution units, and enhance leaching processes.

− HPGR produces better results than attrition scrubbing alone, heavy media and conventional grinding, and enhances flotation and leaching separation processes.

16

EXAMPLES OF APPLICATIONS (CONT.)

• In the Phosphate Industry, HPGR could be used for: − Selective grinding of Dolomitic Phosphate Ores:

o To produce high grade phosphate concentrates.

o To reduce acidulation problems, such as:

High sulfuric acid consumption.

Reactors foaming.

Reduction on P2O5 production.

Filtration and gypsum production problems.

Difficulties in downstream operations and handling.

− Selective Grind of Clayed Phosphate Ores.

• These dolomitic and clayed problems are known to be present in the future Phosphate Florida Reserves.

17

EXAMPLE OF APPLICATIONS (CONT.) Representative Potential Applications - Quantitative Examples.

• Tests on Dolomitic Phosphate Pebbles from four mines and after on pass through a HPGR and desliming were considered with the following results:

• Rod mill grinding for comparison showed a specific energy consumption of 14.46 to 20.55 Kw-h/ton, P2O5 grade of 23.22 to 28.09% and MgO grade of 0.56 to 1.44% with P2O5 recovery at same d50 than HPGR of about 67.66%, and MgO rejection of about 56%. Thus, inferior results on both P2O5 and MgO grades, recovery, and rejection with higher energy consumption.

18

FEED PRODUCTS

Grades Grades Recovery Rejection Sp. Energy

Mine P2O5, % MgO, % d50, µm t10, % P2O5, % MgO, % P2O5, % MgO, % Kwh/ton

I 23.06 0.91 0.26 42.40 22.44 0.46 74.06 61.08 4.19

II 27.60 2.01 0.31 41.20 28.16 0.95 82.22 61.95 4.03

III 23.15 3.85 0.21 48.40 26.20 0.96 77.46 82.87 6.69

IV 24.72 1.99 0.30 34.40 24.50 1.33 80.47 45.69 4.15

EXAMPLES OF APPLICATIONS (CONT.)

Picture of Lab HPGR

19

EXAMPLES OF APPLICATIONS (CONT.) Distribution of values

• Distributions of P2O5 and MgO showed that they were similar

without HPGR grinding; whereas, the distributions were divergent

with HPGR grinding.

20

0.00

10.00

20.00

30.00

40.00

50.00

60.00

70.00

80.00

90.00

100.00

0.01 0.10 1.00 10.00 CU

MU

LA

TIV

E W

EIG

HT

DIS

TR

IBU

TIO

N,

per

cen

t

PARTICLE SIZE, mm

HPGR (HPRM) TEST RESULTS - MINE III

P2O5, As Received MgO, As Received

P2O5, 118 MPa MgO, 118 MPa

EXAMPLE OF APPLICATIONS (CONT.)

• Flotation of the 14x400-mesh fraction of the HPGR product resulted

in 90% of P2O5 recovery with an additional 11% of MgO rejection

using Crago process. For carbonate flotation, a HPGR plus a rod

mill would result in better liberation of dolomite, significant lower

energy consumption, higher recovery of P2O5 and rejection of MgO.

• Selective HPGR grind of clayed phosphate ore prepared at

2x0.020-mm size fraction resulted in:

• Results of in this case showed more P2O5 recovery and Al2O3

rejection. In addition, lower water, and energy consumption were

observed. 21

FEED PRODUCTS

Grades Yield Grades Recovery Rejection Sp. Energy

Process Ore P2O5, % Al2O3, % % P2O5, % Al2O3, % P2O5, % Al2O3, % Kwh/ton

Standard High Clay 20.80 4.10 62.52 20.80 2.14 63.45 69.82 5.40

HPGR High Clay 20.80 4.10 60.00 22.82 2.28 78.82 75.76 3.88

HPGR U.-H. Clay 6.25 11.87 36.34 8.11 4.38 42.45 80.81 1.34

SUMMARY AND CONCLUSIONS • Understanding the mechanism of comminution of High Pressure Grinding

Rolls (HPGR) is of utmost importance to take full advantage of the benefits of this technology.

• Based on the unique possibilities that this technology offers of being able to preferentially grind certain mineral species on a pseudo-confine-autogenous mode, selective grinding is possible for the liberation of troublesome impurities.

• The Bed of Particles Comminution Mechanism of the HPGR resulted in residual stresses and micro-cracks that could enhance downstream attrition scrubbing, leaching processes, and/or reduce energy consumption in fine grinding.

• Consequently, the objective of the process to be designed must be clearly established. Under this condition, full advantage of HPGR technology could be taken for downstream separation processes, such as classification and desliming, gravity separation, magnetic separation, flotation, and leaching.

22